In the field of magnetic recording using a head and a medium, further performance improvement of thin film magnetic heads and magnetic recording media is in demand in association with the high recording density of magnetic disc devices. As a thin film magnetic head, at present, a composite type thin film magnetic head made of a structure where a magnetoresistant (MR) element for reading and an electromagnetic transducer element for writing are laminated is widely used.
The magnetic recording medium is a discontinuous medium in which magnetic grains are aggregated, and each magnetic grain has a single magnetic domain structure. In the magnetic recording medium, one recording bit is configured by a plurality of magnetic grains. Consequently, in order to enhance the recording density, asperities at the border between adjacent recording bits need to be reduced by decreasing the size of the magnetic grains. However, if the magnetic grains are reduced in size, there is the problem that the thermal stability of magnetization of the magnetic grains is reduced in association with a decrease in the volume of the magnetic grain.
As a countermeasure against this problem, an increase of magneto anisotropy energy Ku of magnetic grains may be considered; however, the increase of Ku results in an increase in an anisotropy field (coercive force) of a magnetic recording medium. On the other hand, the upper limit of the recording magnetic field intensity of the thin film magnetic head is substantially determined by saturation magnetic flux density of a soft magnetic material configuring a magnetic core within the head. Consequently, if the anisotropy field of the magnetic recording medium exceeds the acceptable value determined by the upper limit of the recording magnetic field intensity, it becomes impossible to write to the magnetic recording medium. Currently, as a method to solve such a thermal stability problem, a so-called thermally assisted magnetic recording method is proposed in which, while a magnetic recording medium made from a magnetic material with large Ku is used, under a state where the anisotropy field is reduced by heating the magnetic recording medium, a recording magnetic field is applied to record information.
In this thermally assisted magnetic recording method, a method using a near-field light (NF light) probe comprising a metal piece that generates an NF light from plasmon excited by laser light, a so called plasomon generator, is generally known, and as a magnetic head including such a plasmon-generator, a magnetic head including a magnetic pole, a waveguide, a plasmon-generator facing the waveguide and a laser diode as a light source is proposed (U.S. patent application Ser. No. 13/046,117).
In this thermally assisted magnetic head, when laser light is irradiated to a waveguide from a laser diode, the light that has propagated through the waveguide is coupled with the plasmon-generator in the surface plasmon mode to excite surface plasmon. The propagation of this surface plasmon through the plasmon-generator will cause the generation of NF light at a NF light generating portion positioned at an edge part of the plasmon-generator that is opposite to the recording medium surface. The magnetic recording medium is then heated by irradiating the magnetic recording medium with the NF light generated at the NF light generating portion of the plasmon-generator, and information is written by applying a magnetic field in a state where the anisotropy field of the magnetic recording medium is reduced.
In a thermally assisted magnetic disk device including such a thermally assisted magnetic head, in order to accomplish stable magnetic writing with high recording density and a high SN ratio, it is an important factor that the light intensity of the NF light irradiated onto the magnetic recording medium be stable; i.e., the output intensity of laser light from the laser diode as a light source is constantly maintained. However, the output intensity of the laser light from the laser diode can fluctuate due to temperature increase and the like of the laser diode. Consequently, in a thermally assisted magnetic head, pre-shipment inspection to test whether the temperature of the laser diode is at or below the guaranteed operating temperature at the time of driving the laser diode is required.
As a method where such an inspection is implementable, a method is proposed in which the oscillation wavelength of a semiconductor laser is measured and the temperature of the semiconductor laser is calculated from the oscillation wavelength. When the calculated temperature of the semiconductor laser falls outside of the pre-set temperature range, the thermally assisted magnetic head is then determined to be a defective product (JP 2013-97819).